NetworkMenu: remove broken device activation policy

Previously, when toggling a switch on we tried to replicate NM policy and
find a good connection to activate. This is broken in many situations.
Instead, only activate something when we can be sure it's what the user
wants (i.e. when there is only one connection, or when there is none,
and thus connecting will trigger the config dialog)

https://bugzilla.gnome.org/show_bug.cgi?id=683136